For class, I am working on developing a sensor to measure short distances in water. This can hypothetically work at water-level, rather than "underwater", but must be waterproof/or waterproof-able at the very least. It will only need to measure 2m at max, and shouldn't need to withstand more than 2m under water either. There will need to be one receiver and two transmitters, but the sensor type is up for debate. Due to the short distance, I cant imagine using a fish finder or boat depth sensor, and ultrasonic sensors almost seem excessive, too. However, I have been considering ultrasonic/sonar, do to the movement that may be present in the water, which will be pool-water, so I have to account for chemicals in the water as well. An IR or blue/green light seemed possible in still water, but I am unsure with the water conditions. Thank you in advance for any help!
Define "sort distances".
Have you considered a tape-measure? I have a 50metre fabric one I use when diving, or failing that, my DSMB reel line is marked in metres.
2m at max. The receiver will be constantly moving and must be able to recognize when it is 2m away from the transmitter.
Just off the top of my balding old head,,, you need to sync the start of a received signal with the timing routine on the receiver side. remember that you are only dealing with the return signal timing to determine distance. trigger whatever subsequent action on an approximation of two meters.
depending upon distance between transmitters/emitters, you may need to attenuate the received signal to avoid xmitter crosstalk.
Under water only ultrasound (sonar) is possible. Radio waves are out, except MAYBE light as it's a pool so clear water.
Possible option: combination of light and ultrasound. Transmitter produces a bright flash of light, and at the same time transmits an ultrasound ping. Receiver sees the flash, then times how long it takes for the sound to arrive.
Caveats: light sensor needs line of sight to the transmitter, and both light source and light sensor may be directional. Constant movement makes this a challenge.
There may not be ready-made ultrasound transmitters/receivers that can do this. You may have to modify them.
Sound travels much faster in water than in air, around 1,500 m/s, so your 2m distance takes just 1.33 ms. That at least is very much in the timing abilities of Arduinos.